1. Field of Technology to Which the Invention Belongs
The present invention relates to an oscillator with a noise reduction function that reduces the EMI noise of electronic equipment containing an oscillator, a writing device and a control method of a writing device that can change the oscillator setting.
2. Prior Art Technology
In recent years, security measures for electronic equipment such as personal computers and facsimiles are becoming important. In the current situation, products that do not meet the EMI (Electromagnetic Interference) standard cannot be sold in certain locations. The EMI noise is generated by high frequency wave components contained in digital signals radiating from transmission lines. Therefore, in order to reduce the EMI noise, it is important to reduce the high frequency wave components and the loop current.
On the other hand, in modern electronic equipment, due to the demand for high performance and high throughput, the basic operation frequencies of microprocessors exceed 50 MHz and continue to increase even now. However, the EMI noise often increases along with the speed increase of the electronic equipment. Conventionally, when a signal based on the clock signal (various kinds of clock signals and signals on the data bus and clock bus) is a cause of the EMI noise, a method is adopted that eliminates the high frequency wave component of the clock signal etc. with a filter. Also, techniques other than filters for cutting the EMI include shielding in the case shield and the ground plane.
However, because the method of installing a filter requires a space for the filter on the circuit board, there has been a disadvantage for the miniaturization of electronic equipment. Especially in 32-bit microprocessors, because the data bus and address bus occupy a majority of wiring, it is spatially difficult to install filters to the wiring. Also, because there are cases where an EMI test must be passed with an open box due to changes in the EMI standard, there occurs a situation where a method of using a case shield can not be applied. Furthermore, there has been a problem of increased manufacturing costs in the methods of shielding by the case shield and ground plane.
Given such a situation, as shown in FIG. 10, recent oscillators have included a spread spectrum function designed to lower the peak energy of the frequency reference and to reduce EMI noise by spreading the frequency range of the clock signal. By this means, the peak energy of the EMI noise issued from the clock signal and signals derived from the clock signal are lowered, making it possible to meet the EMI standard which regulates the EMI standard value (radiation limit value) at the peak value.
However, because oscillators having this kind of spread spectrum function have their own spread range, spread direction, etc. which is respectively fixed, if the electronic equipment in which an oscillator is embedded does not meet the EMI standard value, the whole oscillator needed to be replaced. In this instance, the spread range refers to the frequency range width of the clock signal treated with spread spectrum modulation (indicated by a code xcex1 in FIG. 10), which include a center spread which permits a spread so that the specification frequency of the oscillator fs becomes the center frequency fc, and a down spread which permits spread so that the frequency based on the specification becomes the maximum frequency. FIG. 10 shows the case of down spread.
Also, for users who embed an oscillator in the electronic equipment, techniques for setting the spread range and spread direction to meet the EMI standard needs to be examined beforehand, and in order to do so, oscillators of different spread ranges etc. need to be manufactured every time or prepared beforehand, which has been a problem.
The present invention has the objective of providing an oscillator with a noise reduction function that can reduce the noise of electronic equipment by arbitrarily changing the spread of the spectrum modulation setting, a writing device that can change the setting of this oscillator, and a control method for the writing device.
In order to solve the problem, the present invention is characterized by, in an oscillator with a noise reduction function, being equipped with an oscillation means that outputs the first reference signal, a memory means where modulation data for performing the spread spectrum modulation are selectively memorized, a write control means that performs writing of the modulation data to the memory means, and rewriting and erasing the modulation data memorized in the memory means according to externally input data, a modulation signal output means that generates modulation signals from the modulation data and outputs them, and a reference signal output means that outputs the second reference signal at a frequency which is a multiple of the frequency of the first reference signal when the modulation signal is not input, or on the other hand outputs the third reference signal wherein the second reference signal is spread spectrum modulated based on the modulation signal when the modulation signal is input.
By means of the present invention, because an oscillator with a noise reduction function generates modulation signals from modulation data memorized in the memory means by the modulation signal output means and outputs a third reference signal spread spectrum modulated based on the modulation signal by the reference signal output means, if the modulation data memorized in the memory means are rewritten, the setting of the spread spectrum modulation can be changed, and if the modulation data are not memorized in the memory means, a second reference signal without spread spectrum modulation can be output.
Also, as a writing device that writes data to the memory means of the oscillation means with a noise reduction function, the present invention is characterized by the fact that it is equipped with a connection means that connects each terminal of the oscillator with a noise reduction function, an input means that inputs the setting information that is to be set to the oscillator with a noise reduction function, a modulation data generation means that, according to the specification of the spread spectrum modulation contained in the setting information, generates corresponding modulation data, a transfer means that transfers the modulation data to the oscillator with a noise reduction function, and a control means that instructs writing of the transferred modulation data to the oscillator with a noise reduction function.
By means of the present invention, according to the input specification of the spread spectrum modulation that is to be applied to the oscillator with a noise reduction function, the writing device can make the modulation data generation means generate corresponding modulation data, and the transfer means writes the data to the memory means of the oscillator with a noise reduction function. Therefore, it becomes possible to arbitrarily set the specification of spread spectrum modulation of the oscillator with a noise reduction function to a specification desired by the user.
Also, in a control method of a writing device that writes data to the memory means of the oscillator with a noise reduction function, the present invention is characterized by having an input procedure that inputs the setting information to be set to the oscillator with a noise reduction function, a modulation data generation procedure that, according to specification of the spread spectrum modulation contained in the setting information, generates the corresponding modulation data, a transfer procedure that transfers the modulation data to the oscillator with a noise reduction function, and an instruction procedure that instructs writing of the transferred modulation data to the oscillator with a noise reduction function.